The present invention relates to a disk reproduction apparatus and, more specifically, to a disk player applicable to different recording/reproduction rates used for audio data, ROM data, etc.
In the field of acoustic equipment, a digital recording/reproduction system for converting an audio signal into a digital signal by PCM (pulse code modulation), recording the digital signal on a recording medium such as a disk and a magnetic tape, and reproducing it therefrom in order to record/reproduce data with high density and high reliability, has recently been known. In particular, a 12-cm-diameter CD (compact disk) for optically reading a bit string corresponding to digitized data is the most wide-spread. In such a compact disk, an optical pickup element including a semiconductor laser and a photoelectric conversion element is moved linearly from the internal circumference of the disk toward the outer circumference thereof, and the disk is rotated at CLV (constant linear velocity), thereby to read data out of the disk. The compact disk stores 16-bit digital data (main information) into which an analog audio signal is converted by PCM. This digital data is stored by repeating one frame constituted of 24 symbols each corresponding to 8 bits.
Data of 32 symbols constituted of digital data of 24 symbols, parity data P of 4 symbols, and parity data Q of 4 symbols, is supplied with 8-bit (one-symbol) sub-code data through a one-frame delay circuit. The 32-symbol data and sub-code data are modulated by EFM (eight to fourteen modulation). A 3-bit margin is formed between the modulated 14-bit symbols, and a 24-bit frame periodic signal is added to the head of the 32 symbols. Thus, 588-bit data is recorded on the disk as one frame and, in this case, since a bit clock is 4.3218 MHz, the data is recorded on the disk at 136 .mu. sec (7.35 KHz) per frame. The sub-code data, which is constituted of sub-code frames each corresponding to 98 frames, is recorded on the disk at 75 Hz (13.3 msec) per sub-code frame.
In the disk reproduction apparatus, digitized data, which is read out from the disk and from which a synchronous signal is separated, is demodulated by EFM and divided into a 32-symbol word component and a sub-code data component.
In a signal processing circuit, the EFM-demodulated data is written to a memory by a reproduction frame clock of a PLL (phase locked loop) circuit and read out from the memory by reference clock of the apparatus, thus absorbing variations in time base due to a disk motor. To change the reproduction velocity of data read out from the disk, a two-fold velocity control signal supplied to a clock circuit for supplying a frame clock for controlling the signal processing circuit, is employed.
Searching for target reproduction data based on time information of sub-code Q data, the same reproduction data can be obtained since the sub-code data and reproduction data have the specific phase relationship on the disk.
A CD-ROM drive as well as a CD is well-known as a typical disk reproduction apparatus. The CD-ROM is a device for reproducing both an audio signal and ROM data such as image information and character codes which exist independently or being mixed in the disk. The audio signal is reproduced at normal reproduction velocity in order to output it as sound, and this velocity is defined as a one-fold velocity. In contrast, the ROM data is reproduced at high velocity, e.g., at an n-fold velocity (n=2 or more) to be read out as quick as possible. In order to reproduce data from such a disk, the reproduction velocity has to be changed frequently, e.g., from one-fold velocity to twofold velocity, and vice versa.
Consequently, if the reproduction of data is intermitted because of a change in velocity, the player is greatly decreased in performance. A high-performance disk motor can be used to improve performance, which greatly increases in costs.
A prior art CD-ROM system (CD-ROM drive) having a CD player will now be described with reference to FIG. 1. In this system, a quartz clock is used for a signal processing reference clock. For buffer RAM access for error correction, a quartz frame clock supplied from a reference clock generation circuit 12 having a quartz oscillator is used for a readout frame clock, and a PLL clock generated by a PLL circuit of a CD signal processing circuit 6 is used for a write frame clock.
A disk 1 is irradiated with a laser beam emitted from a pickup (PU) 3, and its reflected light is received to read information data out of the disk 1. The readout data is supplied to an RF circuit 4 to issue an RF signal having an equalized waveform. The RF signal is converted to binary digital data and called an EFM (eight to fourteen modulation) signal. The EFM signal is input to the PLL circuit, and a sync clock signal CK is extracted from the EFM signal. In the PLL circuit, data signal DATA is generated.
In the signal processing circuit 6, the data signal and clock signal are supplied to a correction circuit through a synchronous separation circuit, and their errors are corrected using a correcting memory (RAM). The corrected data are read out by a quartz type reference clock generated from the reference clock generation circuit 12 based on the clock oscillated by the external quartz oscillator. The correction circuit outputs data signal DATA and sync clock signal CK.
The data signal includes audio data and CD-ROM data. In the audio mode, an audio signal corrected by the correction circuit, is converted to an audio analog signal by a DAC (digital to analog converter) 11. The DAC 11 outputs an audio signal reproduced through a low-pass filter (not shown). In the CD-ROM mode, a data signal is processed by neither the DAC 11 nor LPF, but supplied to a CD-ROM decoder circuit 8 and output as a digital signal. A signal corrected by the correction circuit, is sent from the signal processing circuit 6 to the CD-ROM decoder circuit 8 and transferred to a host computer 10 while executing correction (layered ECC) of a CD-ROM and buffering. In the audio mode, the audio signal is processed by a CD player and thus reproduced at one-fold CLV, whereas in the normal CD-ROM mode, the signal is reproduced at n-fold CLV or CAV.
A servo type signal read out from the pickup 3, is supplied to a servo circuit 5 via the RF circuit 4 and equalized thereby to drive an actuator and a pickup feeding motor of the pickup 3.
The signal processing circuit 6 generates a signal for rotating the disk, and the signal drives a disk motor 2. A system controller 7 controls the servo circuit 5 for controlling various CD servo systems through a microcomputer (I/F).
Since a CD system is originally an audio reproduction system, a CD is rotated at one-fold velocity, but its transfer rate is lower than that of a generally-used computer storage media; therefore, it is an important objective to increase in transfer rate. There has recently been an 24-fold velocity disk reproduction apparatus, and there are no bounds to competition for velocities, such as an increase from twofold velocity to fourfold velocity, eight-fold velocity and twenty four-fold velocity.
In a disk reproduction apparatus, especially a CD-ROM drive having a CD player and a CD-ROM decoder circuit, the reproduction rate of the CD player fixed for audio data is one-fold CLV, whereas in the CD-ROM drive, data is accessed at one or both of higher-velocity n-fold (n&gt;1) CLV and CAV (constant angular velocity), regardless of the reproduction rate fixed for audio data, thus improving in data transfer velocity and search velocity. For audio reproduction, data need to be continuously supplied to a DAC at constant velocity. Since, however, the CD player generally has only a small-sized buffer memory for correcting an error, it was necessary to apply the CD player to the one-fold CLV.
If the velocity of high-velocity reproduction in the CD-ROM mode becomes higher than that of the one-fold velocity reproduction in the audio mode, the circuit constant of the former reproduction is difficult to conform with that of the latter one, because of a servo band, and parallel circuits have to be switched or high-performance (wide band) devices or elements should be adopted, thus increasing in costs.